Glass reinforcement in the form of mat, woven rovings or fabric has been pre-impregnated with thermoset resins and used in subsequent moulding for many years. Usually the matrix resin in these materials has been cured to the `B` stage (an intermediate stage of cure) and the impregnated glass fibre material reeled up between layers of cover film, e.g. polyethylene, to prevent layers sticking to one another. Subsequently the material is unrolled, the interleaving film removed, the `B` stage material cut to the required shape and size and loaded into a mould and cured to the final product by the application of heat and pressure. Attempts were made to cure materials using diallyl phthalate instead of styrene by ultra violet light but were not very successful.
A development in pre-impregnation was the use of chemically thickened unsaturated polyester resins to make sheet moulding compounds ("S M C"). In this process chopped glass fibres or glass mat is impregnated with a mixture of unsaturated polyester resin, filler, catalyst, release agent and a thickening agent such as magnesium oxide. There is a large percentage of filler, this being the cheapest ingredient. Thermoplastic polymers may be added to control shrinkage. The impregnated sheet is compacted and reeled up between layers of polyethylene film. After a short maturing time the material can be unrolled, the polyethylene film removed cleanly and the material cut to size and moulded under heat and pressure. The material in its rolled up state can be comparatively inflexible and unmalleable because it is only more or less roughly cut to size for the mould, and exact conformation to the mould and redistribution of the material of the prepreg will occur under the pressure and heat exerted in the mould.
In DEOS No. 2831679, the invention being described is the provision in a prepreg of special polymeric additives replacing thickeners. These additives are especially provided adjacent the cellophane cover films. The prepreg is stated to have desirable thixotropic properties and to be resistant to air absorption. The prepreg is stated to be cured by sunlight in 1 hour, the photoinitiator used being however an initiater with absorption maxima at 250 and 350 nm, in the ultraviolet. Sunlight as received on the surface of the earth has an ultraviolet content which varies with weather, with air purity and particularly with altitude.
In Example 15 of European Patent Application Publication No. 0007086, published 23rd January, 1980 a reinforced composition containing 40 parts of alumina filler to 60 parts of unsaturated polyester and 1.5% of the resin of MgO thickener is cured by intense exposure to fluorescent tubes. Once more, the initiator used (the object of that invention) is an ultraviolet-sensitive initiator and the process is described as UV curing.
The conclusion drawn is that no preimpregnated material has so far been capable of being cured by light of the visible wavelengths (say 400-800 nm).